1. Field of the Invention
This invention relates to a system for driving a cable and particularly to a system for controlling the normal force applied to a cable as it is being retrieved or deployed.
2. The Prior Art
There are a number of systems currently available for use in driving (i.e., deploying or retrieving) a cable. Certain prior art systems are designed for winding and unwinding electrical cables, and particularly electrical cables used in so-called "towed array" systems. In a towed array system, a set of transducers (e.g., hydrophones) are deployed from a ship to conduct sensing operations (e.g., for military purposes or for oil exploration). A typical towed array system includes a heavy wire cable and a set of transducers electrically connected to the heavy wire cable. Typically, electronics for processing the sensor signals are housed in small cans which are electrically connected to the heavy wire cable. Thus, the cable in a towed array includes a heavy wire cable and relatively fragile electronic equipment housed in small cans. Usually, the cans are enclosed within a hose which is coupled to the heavy wire cable via a metal coupling.
One type of system which is currently available for retrieving and deploying a cable includes a grooved capstan drum and metal pressure rollers for applying a normal force to the cable in order to produce a friction drive force when deploying the cable. The load on the metal pressure rollers is provided by a rubber bushing which acts as a nonlinear torsion spring. For a towed array cable, the applied normal force is the same for the heavy wire cable and the array but rises rapidly at the interface between the hose (housing the electronic cans) and the coupling. Several wraps of the cable are guided in a helical path about the grooved capstan drum by grooved rollers which are spring loaded radially toward the drum to provide the normal force for the frictional drive of the cable when the cable is retrieved.
A second type of cable drive system employs fixed axis urethane grooved rollers and a smooth drum to apply the normal force required for the frictional drive of the cable while deploying and retrieving the cable. The location of the rollers relative to the drum surface and the compliance of the roller and cable material provide the normal force. This force is constant for a given diameter cable, but rises rapidly at the interfaces between the cable and the couplings and drops rapidly with a slight decrease in the diameter of the cable.
One disadvantage of the above-described prior art systems is that they both apply a constant force to a cable of a given diameter. In most operations where the systems are used, a lighter force could be used on the portions of the cable containing fragile electronic equipment during retrieval because the drag is low. In contrast, when retrieving the heavy wire cable portion of the cable (which is generally more rugged) higher forces on the cable can be used because a greater amount of drag can be expected. The above prior art systems compromise on these optimum high and low normal forces by providing a constant normal force which is suitable for driving all portions of the cable.
Another disadvantage of the above systems is that when a coupling (which is a rigid metal tube of predetermined length) engages the drum in either of the above systems, the interface of the cable and the coupling is subjected to a bend radius which is considerably smaller than the drum radius. This interface bending also occurs between the drum radius and the smaller bend radius as the coupling engages each pressure roller. The amplitude and frequency of bending depend upon the coupling length and roller spacing.
As a result of the constant normal force which is applied in both of the above-described prior art systems, failures may occur because the fragile electronics present in the hose portion of the cable may break if a large normal force is applied to this portion of the cable, and because the cable may be broken at one or more of the interfaces between the cable and the coupling due to the above-described bending radius. In addition, the above-described second prior art system may produce a failure if the cable cold flows while stored on the drum. This is because stress relaxation will reduce the normal force to zero, thereby reducing the frictional drive force to zero. As a result, when it is desired to restart the system to again drive the cable, there will be no frictional drive force to drive the cable.
In summary, there is a need in the art for a cable drive system which is capable of providing a sufficient normal force to drive the cable at all times but without providing a force which will damage fragile components or coupled portions of the cable.